1. Technical Field
The present disclosure relates to an arc tube for discharge bulb, including a sealed glass bulb where electrodes are provided in a sealed glass bulb so as to face each other and luminous materials are enclosed.
2. Related Art
In an arc tube disclosed in JP-A-11-067153 and JP-A-2003-86136, electrodes are provided in a sealed glass bulb so as to face each other. The sealed glass bulb is formed by pinch-sealing regions of an electrode assembly, where molybdenum foils are present, by quartz glass and sealing luminous materials and the like. In the electrode assembly, electrode rods, molybdenum foils, and lead wires are integrally connected in series. If the molybdenum foils at high temperature are recrystallized and crystal grains of the molybdenum foils are roughened when a glass tube is to be pinch-sealed, the value of stress, which is generated at the interface between the molybdenum foil and quartz glass by the turning on/off of the arc tube, is increased. For this reason, the molybdenum is likely to peel off from foil glass, which causes a sealing gas to leak at a pinch-sealed portion. In JP-A-11-067153 and JP-A-2003-86136, a technique for solving the above problem has been described.
In JP-A-11-067153, the size of the recrystallized grains of the molybdenum foil at the pinch-sealed portion is set to 50 μm or less, so that stress repeatedly generated at the interface between the molybdenum foil and quartz glass by the turning on/off of an arc tube is reduced.
In JP-A-2003-86136, the surface of a molybdenum foil at a pinch-sealed portion is formed of a roughened surface, which is subjected to an etching treatment including oxidation and reduction, to increase adhesion (mechanical bond strength) between molybdenum foil and glass. Accordingly, even though stress is repeatedly generated at the interface between the molybdenum foil and quartz glass by the turning on/off of an arc tube, the molybdenum foil is not likely to peel off from glass.
In the disclosure of JP-A-11-067153 and JP-A-2003-86136, it is advantageous in preventing the molybdenum foil from peeling off from the glass at the pinch-sealed portion to some extent. However, in manufacturing the arc tube disclosed in JP-A-11-067153 and JP-A-2003-86136, the inventor has produced a prototype of an arc tube using a molybdenum foil that is subjected to a vacuum heat treatment (about 900° C.) or a hydrogen treatment (about 900° C.) (a molybdenum foil whose the crystal grain size is in a range of about 1 to 1.5 μm by a primary recrystallization, instead of a molybdenum foil that is subjected to an etching treatment including oxidation and reduction (a molybdenum foil having a rough surface) as the molybdenum foil (the molybdenum foil used for connecting the electrode rod with the lead wire) of the electrode assembly. Consequently, when pinch-sealing is performed at a relatively high temperature, it was possible to suppress the molybdenum foil from peeling off from the glass, which causes a sealing gas to leak at a pinch-sealed portion, while the life of the arc tube is lengthened.
That is, it was possible to obtain a result that the separation between glass and the molybdenum foil is suppressed even though the surface of the molybdenum foil at the pinch-sealed portion does not have an extremely roughened surface structure for increasing the adhesion (mechanical bond strength) between the molybdenum foil and glass.
Accordingly, the inventor carefully observed the cross-section of the molybdenum foil of this prototype in order to look into this cause. As a result, a plurality of closed cavities, which had not been seen in a comparative example (an arc tube using a molybdenum foil that is not subjected to a primary recrystallization treatment) at all, was seen in the molybdenum foil. That is, closed cavities (cavities reliably separated from a glass layer) are formed in the molybdenum foil at the pinch-sealed portion. Accordingly, it is confirmed that the cavities (a porous structure of the molybdenum foil including a plurality of closed cavities) reduce stress generated at the interface between the molybdenum foil and quartz glass when the arc tube is turned on/off and are advantageous in suppressing the occurrence of the peeling off of molybdenum from glass.
In addition, even in the case of the arc tube of JP-A-2003-86136 (a roughened surface structure where the surface of the molybdenum foil at the pinch-sealed portion increases the adhesion between the glass and molybdenum foil), when the molybdenum foil (a molybdenum foil whose crystal grain size is in a range of about 1 to 1.5 μm by a primary recrystallization) subjected to an oxidation (about 500° C.)/reduction (about 900° C.) treatment, which is a primary recrystallization treatment, is pinch-sealed at a relatively high temperature, closed cavities are formed in the molybdenum foil. Accordingly, it is confirmed that the adhesion (mechanical bond strength) between the glass and the molybdenum foil is increased by the rough surface structure of the surface of the molybdenum foil and a porous structure in the molybdenum foil is advantageous in suppressing the occurrence of the peeling off of molybdenum from glass.